The present invention relates to a system for controlling the distribution of a driving force or torque between front wheels and rear wheels of a four-wheel drive vehicle.
Japanese patent provisional publication No. 58-26636 shows a conventional example of such a control system. In this system, a control circuit calculates a slip rate between front and rear wheels from front wheel and rear wheel speeds sensed by rotational speed sensors, and fully engages a clutch in a transfer case to distribute a driving torque equally between the front and rear wheels when the slip rate is equal to or greater than a predetermined value. Thus, this system controls the driving torque distribution in accordance with the difference between the front wheel speed and the rear wheel speed. In this system, however, a front and rear wheel speed difference due to skid such as spin (spin-out) and drift (drift-out) is not taken into account.